1. Field of the Invention
The present disclosure relates to a signal processing circuit and a signal processing method suitable for use with capacitive position detection sensors capable of detecting positions pointed to by a plurality of pointers such as active capacitive pens, and to a position detector and an electronic device having the same.
2. Description of the Related Art
Position detectors such as touch panel have come into wide use, prompting various inventions relating to position detectors to be developed. For example, Patent Document 1 (Japanese Patent Application Laid-Open Publication No. 2011-243081) includes a disclosure relating to a capacitive touch panel device. In the case of a position detector disclosed in Patent Document 1, a panel main body 4 (position detection sensor) is formed by arranging a plurality of transmitting electrodes (transmitting conductors) 2 and a plurality of receiving electrodes (receiving conductors) 3 in a grid pattern so as to supply a given signal to the transmitting electrodes 2. At a position pointed to by a finger serving as a pointer, a current (charge) is divided via the finger, thus changing a capacitance (mutual capacitance) formed between the transmitting electrode 2 and the receiving electrode 3. The change in current through the receiving electrode 3 based on this change in capacitance is measured.
Therefore, it is possible to detect the position of the panel main body 4 pointed to by the pointer by detecting the change in current based on the change in mutual capacitance at each of the intersections between the transmitting electrodes 2 and the receiving electrodes 3.
However, the current that changes in the receiving electrode 3 at a position pointed to by a pointer such as a finger is extremely weak. Therefore, such an extremely weak current is converted to a voltage or current of an appropriate signal level for processing. The above Patent Document 1 describes the conversion of an extremely weak current flowing through the receiving electrode 3 to a voltage with an IV conversion section (current-voltage or charge-voltage conversion section) 31 that uses an op-amp (operational amplifier) OPA (FIG. 5).
Further, a position pointer called active capacitive pen is known as a pointer. Among known types of active capacitive pens are that which includes a transmitter and supplies a transmission signal to a position detector and that which receives a signal from a position detector, amplifies the received signal, and supplies the amplified signal to the position detector. The position detector is designed to receive a signal from the active capacitive pen with a sensor electrode (conductor) through electric field coupling and make a determination of the received signal for each electrode (conductor), thus detecting the position pointed to by the active capacitive pen.
A touch panel device including an IV conversion section as disclosed in the above Patent Document 1 is not suitable for use with mobile devices called smartphones that have quickly become pervasive in recent years. A smartphone includes, for example, a display screen of about four inches and detects a position pointed to by a pointer, such as pen or a finger of a user, through a touch panel device (position detector) arranged on the display screen. A smartphone faces demands for lower power consumption, downsizing, and reduction in weight as a mobile device.
However, the IV conversion section, which commonly includes a capacitor and a resistor between the input and output of the op-amp as illustrated in FIG. 5 of the above Patent Document 1, consumes a lot of power because the op-amp is used to convert a current to a voltage. Further, the IV conversion section requires a capacitor having a relatively large capacitance. In order to form a capacitor using a semiconductor process during integration of the IV conversion section into an IC chip, a semiconductor area over which the capacitor is formed is considerably larger than those of other circuit elements, making it difficult to integrate the IV conversion section into an IC chip.
In the touch panel device described in the Patent Document 1, a plurality of receiving electrodes share the single IV conversion section 31. One of the plurality of receiving electrodes is connected to the single IV conversion section 31 via a switching circuit 21 for current-to-voltage conversion.
However, if the plurality of receiving electrodes share the single IV conversion section 31, a pointed position may not be detected in a timely manner depending on the relationship between the processing speed for switching the plurality of receiving electrodes (receiving conductors) from one to another for connection to the IV conversion section 31 and current-to-voltage conversion and the speed at which the pointer moves on the touch panel. In this case, the detection of a position pointed to by the pointer at an appropriate time may be overlooked.
To improve the above problem, the present applicant proposed a signal processing circuit as Japanese Patent Application No. 2012-222472 (filed on Oct. 4, 2012). This circuit has no IV converter made up of an op-amp, a capacitor, and a resistor for current-to-voltage conversion and receives, with a capacitor circuit, the change in voltage obtained by the receiving conductors. The capacitor circuit obtains, as a voltage signal, the change in capacitance in response to pointing of a position by the pointer. The position detector using the signal processing circuit according to the previous application consumes small power and can be reduced in circuit scale, making it suitable for use with mobile devices such as smartphones.
If an extremely weak current flowing through a receiving conductor is converted to a voltage using the IV conversion section for processing as in the Patent Document 1, all the currents flowing through the receiving conductors flow into the capacitor connected between the input and output of the op-amp (capacitor in the IC). As a result, the position detection sensor including the transmitting and receiving conductors remains unaffected by the self-capacitances of the receiving conductors.
However, if the capacitor circuit obtains, as a voltage signal, the change in capacitance in response to pointing of a position by the pointer, as used in the signal processing circuit proposed in the previous application, the capacitor circuit is affected by the self-capacitances of the receiving conductors of the position detection sensor.
That is, in the case of a position detection sensor adapted to detect the change in capacitance (mutual capacitance) formed between the transmitting electrode 2 and the receiving electrode 3 as a result of division of the current (charge) via the finger as in Patent Document 1, a voltage change V taking place in the receiving conductor pointed to by the finger is proportional to −Q/(Cx+Co) where −Q is the charge divided via the finger, Cx is the self-capacitance of the receiving conductor, and Co is the capacitance of the capacitor circuit. As a result, the voltage change V is affected by the self-capacitance Cx of the receiving conductor.
Similarly, in the case of an active capacitive pen, the voltage change V taking place in the receiving conductor pointed to by the active capacitive pen is proportional to +Q/(Cx+Co) where +Q is the charge applied to the receiving conductor via the active capacitive pen. As a result, the voltage change V is affected by the self-capacitance Cx of the receiving conductor.
Therefore, when a pointing input is made to the position detection sensor by a pointer such as a finger (when a finger approaches (hovers over) or touches the position detection sensor), the self-capacitance of the receiving conductor of the position detection sensor increases. However, this increase in self-capacitance causes the voltage signal corresponding to a position pointed to by the pointer to diminish, thus changing a detection sensitivity of the pointer.